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Abstract:

Embodiments of the present invention provide methods for edge film stack
removal for use in fabricating photovoltaic devices. In one embodiment,
the method includes providing a substrate having a film stack deposited
thereon, the film stack comprising a transparent conductive layer, a
silicon-containing layer, and a metal back contact layer, removing the
metal back contact layer and the silicon-containing layer formed on a
periphery region along a side of the substrate using an electromagnetic
radiation delivered at a first energy level, and removing the transparent
conductive layer formed on the periphery region along the side of the
substrate using electromagnetic radiation delivered at a second energy
level that is higher than the first energy level.

Claims:

1. A method for processing solar cell devices on a substrate, comprising:
providing a substrate having a film stack deposited thereon, the film
stack comprising a transparent conductive layer, a silicon-containing
layer, and an electrically conductive layer; removing the electrically
conductive layer and the silicon-containing layer formed on a periphery
region along a side of the substrate using an electromagnetic radiation
delivered at a first energy level; and removing the transparent
conductive layer formed on the periphery region along the side of the
substrate using electromagnetic radiation delivered at a second energy
level that is higher than the first energy level.

2. The method of claim 1, wherein the electromagnetic radiation is
directed through the substrate from the bottom to a desired location in
one or more of the layers to be removed.

4. The method of claim 1, wherein the electromagnetic radiation used to
remove the electrically conductive layer and the silicon-containing layer
is delivered at an energy density of about 15 mJ/mm2 to about 75
mJ/mm.sup.2.

5. The method of claim 1, wherein the electromagnetic radiation used to
remove the transparent conductive layer is delivered at an energy density
of about 45 mJ/mm2 to about 120 mJ/mm.sup.2.

6. The method of claim 4, wherein the electromagnetic radiation used to
remove the electrically conductive layer and the silicon-containing layer
is delivered by directing a series of sequential laser pulses at a spot
overlap of about 2% to about 15%, a line overlap of about 5% to about
15%, and a segment overlap of about 15 to about 45%.

7. The method of claim 5, wherein the electromagnetic radiation used to
remove the transparent conductive layer is delivered by directing a
series of sequential laser pulses at a spot overlap of about 10% to about
30%, a line overlap of about 5% to about 20%, and a segment overlap of
about 15% to about 45%.

8. The method of claim 1, wherein the silicon-containing layer is a film
stack comprising a p-type silicon containing layer, a n-type silicon
containing layer, and an intrinsic type silicon containing layer
sandwiched between the p-type and n-type silicon containing layers.

11. A method for processing solar cell devices on a substrate,
comprising: providing a substrate having a transparent conductive layer
and a film stack deposited over the transparent conductive layer, the
film stack comprising one or more silicon-containing layers and an
electrically conductive layer; removing the film stack from a periphery
region along a side of the substrate during a first scan of an
electromagnetic radiation line delivered at a first power level; and
removing the transparent conductive layer from the periphery region along
the side of the substrate during a second scan of the electromagnetic
radiation line delivered at a second power level that is different from
the first power level.

12. The method of claim 11, wherein the first power level is lower than
the second power level.

13. The method of claim 11, wherein the electromagnetic radiation line is
delivered by directing a series of sequential laser pulses.

14. The method of claim 11, wherein the electromagnetic radiation line
and the substrate are moved relative to each other to process the
periphery region of the substrate to be removed.

15. The method of claim 13, wherein the electromagnetic radiation used to
remove the film stack is delivered at a spot overlap of about 2% to about
15%, a line overlap of about 5% to about 15%, and a segment overlap of
about 15 to about 45%.

16. The method of claim 13, wherein the electromagnetic radiation used to
remove the transparent conductive layer is delivered at a spot overlap of
about 10% to about 30%, a line overlap of about 5% to about 20%, and a
segment overlap of about 15% to about 45%.

17. A method for processing solar cell devices on a substrate,
comprising: providing a substrate having a transparent conductive layer
and a film stack sequentially deposited thereon, the film stack
comprising one or more silicon-containing layers and an electrically
conductive layer; directing a series of sequential laser pulses delivered
at a first pulse energy across a periphery region along a side of the
substrate to remove the film stack; and directing a series of sequential
laser pulses delivered at a second pulse energy across the periphery
region along the side of the substrate to remove the transparent
conductive layer, wherein first pulse energy is lower than the second
pulse energy.

18. The method of claim 17, wherein the series of sequential laser pulses
are partially overlapped to provide a laser ablating segment covering at
least a portion of the periphery region.

19. The method of claim 18, wherein the laser ablating segment is
repeatedly applied onto the periphery region along the side of the
substrate in a single traverse over the substrate.

20. The method of claim 19, wherein the laser ablating segment are
repeated at a segment overlap of about 15 to about 45%.

[0003] The present invention relates to methods for an edge film removal
process, more particularly, for an edge film removal process for
fabricating photovoltaic devices.

[0004] 2. Description of the Background Art

[0005] Photovoltaic (PV) devices or solar cells are devices which convert
sunlight into direct current (DC) electrical power. Typically, a thin
film solar cell includes a photoelectric conversion unit and a
transparent conductive layer. The transparent conductive layer is
disposed as a front electrode on the bottom of the solar cell in contact
with a glass substrate and/or as a back surface electrode on the top of
the solar cell. The photoelectric conversion unit may include a p-type
silicon layer, a n-type silicon layer and an intrinsic type (i-type)
silicon layer sandwiched between the p-type and n-type silicon layers.
When the p-i-n junction of the PV cell is exposed to sunlight (consisting
of energy from photons), the sunlight is directly converted to
electricity through a PV effect.

[0006] During deposition, the material being deposited may deposit on, and
over, the edge of a substrate, resulting in an electrical connection, or
"short", between the two transparent conductive layers. Therefore, the
continuity of material from the front to the back, over the side of the
substrate, must be broken to eliminate the short. In some cases, a shadow
mask or edge ring may be used to mask the edge from the depositing
material, but this can result in non-uniform deposition at the substrate
edge regions, as compared to other layers deposited without such masks or
using overlying frames having a different alignment with respect to the
substrate edge as compared to the shadow frame. Additionally, material
along the edge of the substrate, which if left intact may interfere with
the assembly and packaging of the cells into a solar panel frame, must
also be removed to eliminate interference or other potential issues
during assembly into a frame.

[0007] In conventional edge isolation techniques, material removal with a
diamond impregnated belt or with a grinding wheel is typically used to
mechanically grind unwanted edge residuals or deposited materials from
the periphery regions of the substrate. However, these techniques often
result in incomplete material removal on the substrate edge, as well as
scratches or even substrate damage such as micro cracks at the substrate
edge. Alternatively, a thermal ablation process using a single-pass,
high-energy laser has been used by the industry to remove unwanted
materials from the substrate periphery region with better throughput than
mechanical removal approaches without leaving scratches or micro cracks
at the substrate edge. However, with a larger number of thin film layers
as used in a tandem junction design, it has been reported that the
conventional single-pass laser edge removal process may still leave
powder residues and visible debris at the periphery region of the
substrate, which may impact long-term cell and power reliability. Thus, a
higher energy laser may be needed to ablate the thicker tandem junction,
but the use of a high-energy laser for removing thicker film stacks at
the periphery region of the tandem junction solar cell may damage the
glass substrate or change film properties of the transparent conductive
film when passing therethrough. Furthermore, it has been also found that
the high-energy laser would inevitably melt the back metal contact layer
and the silicon-containing film stack deposited on the transparent
conductive film and form an unwanted alloy that is difficult to remove.

[0008] Therefore, there is a need for an improved laser edge removal
process for effectively removing film materials from the periphery region
of solar cells without having issues as discussed above.

SUMMARY OF THE INVENTION

[0009] The present invention provides methods for edge film stack removal
for use in fabricating photovoltaic devices. In one embodiment, a method
for processing solar cell devices on a substrate includes providing a
substrate having a film stack deposited thereon, the film stack
comprising a transparent conductive layer, a silicon-containing layer,
and an electrically conductive layer, removing the electrically
conductive layer and the silicon-containing layer formed on a periphery
region along a side of the substrate using an electromagnetic radiation
delivered at a first energy level, and removing the transparent
conductive layer formed on the periphery region along the side of the
substrate using electromagnetic radiation delivered at a second energy
level that is higher than the first energy level.

[0010] In another embodiment, a method for processing solar cell devices
on a substrate includes providing a substrate having a transparent
conductive layer and a film stack deposited over the transparent
conductive layer, the film stack comprising one or more
silicon-containing layers and an electrically conductive layer, removing
the film stack from a periphery region along a side of the substrate
during a first scan of an electromagnetic radiation line delivered at a
first power level, and removing the transparent conductive layer from the
periphery region along the side of the substrate during a second scan of
the electromagnetic radiation line delivered at a second power level that
is different from the first power level.

[0011] In yet another embodiment, a method for processing solar cell
devices on a substrate includes providing a substrate having a
transparent conductive layer and a film stack sequentially deposited
thereon, the film stack comprising one or more silicon-containing layers
and an electrically conductive layer, directing a series of sequential
laser pulses delivered at a first pulse energy across a periphery region
along a side of the substrate to remove the film stack, and directing a
series of sequential laser pulses delivered at a second pulse energy
across the periphery region along the side of the substrate to remove the
transparent conductive layer, wherein first pulse energy is lower than
the second pulse energy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] So that the manner in which the above recited features of the
present invention are attained and can be understood in detail, a more
particular description of the invention, briefly summarized above, may be
had by reference to the embodiments thereof which are illustrated in the
appended drawings.

[0014]FIG. 2A depicts a top view of the substrate prior to processing in
the laser edge removal tool;

[0015]FIG. 2B depicts a cross-sectional view of a substrate with solar
cell devices and residual films formed in the periphery region of the
substrate;

[0016]FIG. 3 depicts a side view of a laser edge removal tool that may be
utilized to remove films from the periphery region of the substrate; and

[0017]FIG. 4 depicts a cross-sectional view of a periphery region of a
substrate on which a laser edge removal process has been performed.

[0018] To facilitate understanding, identical reference numerals have been
used, where possible, to designate identical elements that are common to
the figures. It is contemplated that elements and features of one
embodiment may be beneficially incorporated in other embodiments without
further recitation.

DETAILED DESCRIPTION

[0019] Embodiments of the present invention provide methods for edge film
stack removal process for fabricating photovoltaic devices. Particularly,
the present invention provides a multi-pass laser edge removal process
for thin film solar devices which typically include a front electrode
layer, a silicon-containing film stack, and a metal back electrode layer
that are sequentially deposited over the substrate. The multi-pass laser
edge removal process may include a first laser pass using a lower power
energy directed to remove the metal back electrode layer and the
silicon-containing film stack, and a second laser pass using a higher
power energy directed to remove the front electrode layer (e.g., a TCO
layer). The energy used for the first laser pass is relatively low so
that it will not melt the metal back electrode layer and the
silicon-containing film stack on the back and/or change film properties
of the front electrode layer when passing through the front electrode
layer. The present invention advantageously increases clearness and
accuracy of removing film stacks at a periphery region along a side of a
substrate without having issues with residue powders or formation of
unwanted alloys due to melting of the back metal contact and
silicon-containing materials, thereby providing a good seal surface for
the substrate to facilitate subsequent bonding and packaging processes
and eliminating shorts across the active energy converting region of the
substrate.

[0020]FIG. 1 depicts a process sequence 100 for manufacturing solar cell
devices on a substrate. The process sequence 100 may include a plurality
of process steps performed in different processing modules and automation
equipment for manufacturing the solar cell devices. It is noted that FIG.
1 only depicts a portion of the process steps performed during the
manufacture of solar cell devices. The configurations, number of
processing steps, or order of the processing steps in the process
sequence 100 is exemplary, and not intended to limit the scope of the
invention described herein. Some other process steps of the process
sequence are known to those skilled in the art and eliminated for sake of
brevity. One suitable example of the overall process sequence is
disclosed in detail by U.S. application Ser. No. 12/202,199, filed Aug.
29, 2008 by Bachrach et al., titled "Photovoltaic Production Line", and
is incorporated herein by reference.

[0021] As shown in FIG. 1, a process sequence 100 starts at step 101 by
loading a substrate into a solar cell production line. The production
line may include a plurality of processing tools and automation equipment
to facilitate fabricating the solar cells on the substrate. FIG. 2B is
completion of a solar cell device showing the layers formed thereon. The
sequence will be described below with reference to steps shown in FIG. 1
and layers shown in FIG. 2B. At step 102, a first transparent conductive
oxide (TCO) layer 214, which may serve as a front electrode in the solar
cell device, is deposited on the substrate 202 in a deposition chamber
such as a PVD chamber. The first TCO layer 214 has scribed therein a
pattern of first scribe lines 220A (only one is shown). The first TCO
layer 214 may be a zinc containing material, aluminum containing
material, tin containing material, ITO containing material, alloys
thereof, or any other suitable conductive materials such as cadmium
stannate. In one example, the first TCO layer 214 is a metal oxide such
as tin oxide, zinc oxide, indium tin oxide, or combinations thereof. The
first TCO layer 214 may also include additional dopants and components.
For example, the zinc oxide may further include dopants that are selected
from a group consisting of aluminum containing materials, boron
containing materials, titanium containing materials, tantalum containing
materials, tungsten containing materials, alloys thereof, combinations
thereof, gallium, or other suitable dopants, depending upon the
application.

[0022] At step 104, a silicon-containing film layer 216 is deposited over
the first TCO layer 214 as shown in FIG. 2B to produce the active, energy
converting portion of the solar cell. The silicon-containing film layer
216 may be a film stack including a p-type silicon containing layer, a
n-type silicon containing layer, and an intrinsic type (i-type) silicon
containing layer sandwiched between the p-type and n-type silicon
containing layers. Multiple layers may be formed in the
silicon-containing film layer 216 for different process purposes. For
example, multiple silicon-based layers of n-i-p, p-i-n, n-p or p-n layers
may be used in the silicon-containing film layer 216 to provide one or
more, e.g., multiple, junctions to improve light conversion efficiency.
Suitable examples of the silicon-containing film stack are disclosed in
U.S. application Ser. No. 11/624,677, filed Jan. 18, 2007 by Choi et al,
titled "Multi-Junctions Solar Cells and Methods and Apparatus for Forming
the Same", U.S. application Ser. No. 12/208,478, filed Sep. 11, 2008 by
Sheng et al, titled "Microcrystalline Silicon Alloys for Thin Film and
Wafer Based Solar Applications", and are herein incorporated by
reference.

[0023] At step 106, an interconnect formation process is performed to form
an interconnect, such as second scribing lines 220B, in the
silicon-containing film layer 216 as shown in FIG. 2B. The interconnect
formation process is performed to electrically isolate various regions on
the substrate of the solar cell from each other by a laser ablation
process using, for example, a Nd:vanadate (Nd:YVO4) laser source. It
is noted that the interconnect formation process can be performed during
different stages of the process sequence 100 to form different scribing
lines 220A, 220B, 220C that electrically isolate adjacent solar cells.

[0024] At step 108, a metal back contact layer 218 is deposited over the
silicon-containing film layer(s) 216 as shown in FIG. 2B, or optionally
deposited over a second transparent conductive oxide (TCO) layer (not
shown) which may be similar to the first TCO layer 214 and serves as a
back electrode. The metal back contact layer 218 may include, but is not
limited to, a material selected from the group consisting of Al, Ag, Ti,
Cr, Au, Cu, Pt, alloys thereof, or combinations thereof. After depositing
the metal back contact layer 218, a pattern of third scribe lines 220C is
formed through the metal back contact layer 218 and the
silicon-containing film layer 216, thereby electrically connecting the
back metal contact layer 218 through the silicon-containing film layer
216 to the front electrode (i.e., the TCO layer 214) of the adjacent
cells. While not mentioned here, other films or materials may be provided
over metal back layer 218 to complete the solar cell. The solar cells may
be interconnected to form modules, which in turn can be connected to form
arrays for a higher performance.

[0025] At step 110, an optional quality assurance and/or shunt removal
process may be performed on the substrate 202 to assure that the devices
formed on the substrate surface meet a desired quality standard and in
some cases correct defects in the formed devices. During the testing
process, a probing device is used to measure the quality and material
properties of the formed solar cell device by use of one or more
substrate contacting probes. In one embodiment, the quality assurance
testing tool projects a low level of light at the p-i-n junction(s) of
the solar cell and uses the one more probes to measure the output of the
cell to determine the electrical characteristics of the formed solar cell
device(s). If the module detects a defect in the formed device,
corrective action may be taken to fix the defects. For example, if a
short or other similar defect is found, it may be desirable to create a
reverse bias between regions on the substrate surface to control and/or
correct one or more of the defectively formed regions of the solar cell
device. The reverse bias generally delivers a high voltage sufficient to
cause the conductive elements in areas between the isolated regions to
change phase, decompose or become altered in a way that eliminates or
reduces the magnitude of the electrical short, thereby correcting the
defects in the solar cells.

[0026] At step 112, after the optional quality assurance and/or shunt
removal process, the substrate 202 is transferred to a laser edge removal
tool to remove a portion of the film stack formed at the periphery region
of the substrate 202. The substrate 202 is positioned in the laser edge
removal tool to remove a portion of the film stack along the edge of the
substrate to reduce the likelihood of damage, such as clipping or
particle generation, from occurring during subsequent processing.
Additionally, removal of the edge portion of the film stack may also
leave the periphery region of the substrate 202 free of materials which
can be utilized later for a frame holding area to facilitate bonding or
sealing the substrate 202 to a backside of another substrate to complete
the solar cell module assembly. The laser edge removal process in
accordance with the present invention will be discussed in detail below
in conjunction with FIGS. 3 and 4.

[0027] At step 114, end of the line processes are performed on the
substrate 202. End of the line processes may include final wire
attaching, bonding, packaging, and backside substrate bonding processes.
At step 116, after the support structure, wiring structures, or framing
structures are formed on the substrate, the substrate 202 is removed from
the production line and the solar cell fabrication process is completed.
It is noted that some other steps may be performed in between each steps
to manufacture the devices. The process sequence 100 only provides an
exemplary process sequence that includes only a portion of some major
process steps to manufacture the devices. It is contemplated that other
process sequences associated with the solar cell device fabrication may
also be adapted to use the laser edge removal process of the present
invention as will be descried below.

[0028]FIG. 2A is a top view of the substrate 202 prior to processing in
the laser edge removal tool at step 112. As discussed above, after layers
of deposition, the periphery region 210 of the substrate 202 may have a
different film stack thickness than the thickness of the film stack in
the cell integrated region 212. In certain cases, the periphery region
210 may have a width 208 ranging between about 8 mm and about 15 mm, such
as about 12 mm, from the substrate edge. As discussed above, when forming
different layers on the substrate utilizing different tools, a mismatch
in the film thickness may result at the periphery region 210 of the
substrate 202 due to the use of the shadow mask during the
silicon-containing film stack deposition that would cause the substrate
periphery region 210 to be free of the silicon-containing film layer 216.
This is because the shadow frame may not fully cover the periphery region
210 even if with the shadow frame, or the silicon-containing film layer
216 may accidentally accumulate or deposit on a portion of the first TCO
layer 214, resulting in the silicon-containing film layer 216' partially
deposited on the periphery region 210. Therefore, the film thickness at
the periphery region 210 may include the thickness of the first TCO layer
214 and the metal back contact layer 218, and a portion of the
silicon-containing film layer 216' sandwiched between the first TCO layer
214 and the metal back contact layer 218 from the side, as shown in FIG.
2B. In order to remove the residual films formed in the periphery region
210 of the substrate 202, the substrate 202 is further transferred to the
laser edge removal tool as discussed above at step 112 to remove unwanted
films in the periphery region 210 using the inventive laser edge removal
process discussed below in conjunction with FIG. 3.

[0029]FIG. 3 shows a side view of an exemplary laser edge removal tool
300 that may be used to remove one or more of films from the periphery
region 210 of the substrate 202. The laser edge removal tool 300
generally includes a wave electromagnetic radiation module 306, a stage
302 configured to receive and maneuver the substrate 202 disposed
thereon, and a translation mechanism 316. In one embodiment, the wave
electromagnetic radiation module 306 is positioned beneath the substrate
202 and opposite an exhaust mechanism or particle collector 304 used for
extracting material ablated or otherwise removed from the substrate
during the process. In another embodiment, the wave electromagnetic
radiation module 306 may be positioned above the substrate 202 to ablate
the film stack at the periphery region 210 of the substrate 202 from the
top. Alternatively, the wave electromagnetic radiation module 306 may be
positioned beneath the substrate 202 while flipping the substrate 202
during the laser edge removal process.

[0030] The wave electromagnetic radiation module 306 comprises a wave
electromagnetic radiation source 308 and focusing optics 310 disposed
between the wave electromagnetic radiation source 308 and the stage 302.
In one embodiment, the wave electromagnetic radiation source 308 may be
an infrared (IR) laser beam source, a Nd:YAG or Nd:YVO4 laser beam
source, crystalline disk laser source, fiber-diode (fiber laser) or other
suitable laser beam source that can provide and emit a pulsed or
continuous wave of radiation at a wavelength between about 1030 nm and
about 1070 nm, such as about 1064 nm, to ablate materials from the
substrate surface. In another embodiment, the wave electromagnetic
radiation source 308 may include multiple laser diodes, each of which
produces uniform and spatially coherent light at the same or desired
wavelength. The power of the laser diode/s is in the range of about 100
Watts to about 1000 Watts.

[0031] The focusing optics 310 may include one or more collimators to
collimate radiation from the wave electromagnetic radiation source 308
into a substantially parallel beam. This collimated radiation beam is
then focused by at least one lens 320 into a line of radiation 312
directed at the periphery region 210 of the substrate 202. Alternatively,
a beam profiler (not shown) may be used where scanners are used, allowing
for calibration of the beam and/or adjustment of beam position. The
radiation 312 is focused on the periphery region 210 along the side of
the substrate 202 to remove the film stack from the periphery region 210.
The radiation 312 emitted from radiation source 308 may scan around each
side of the substrate 202 as many times as needed, or using the inventive
laser edge removal process described below, until the film stack has been
completely removed.

[0032] Lens 320 may be any suitable lens, or series of lenses, capable of
focusing radiation into a line or spot. In one embodiment, lens 320 is a
cylindrical lens. Alternatively, lens 320 may be one or more concave
lenses, convex lenses, plane mirrors, concave mirrors, convex mirrors,
refractive lenses, diffractive lenses, Fresnel lenses, gradient index
lenses, or the like.

[0033] A particle collector 304 may be disposed adjacent to the periphery
region 210 of the substrate, depending upon the location of the wave
electromagnetic radiation module 306. During laser cutting, the material
being removed may be drawn to the particle collector 304 to maintain
cleanness of the tool 300. The stage 302 can be any platform or chuck
capable of securely holding the substrate 202 during transmission. In one
aspect, the stage 302 includes a means for grasping the substrate, such
as a frictional, vacuum, gravitational, mechanical, or electrical system.
Examples of suitable means for grasping may include, but are not limited
to, mechanical clamps, electrostatic or vacuum chucks, or the like.

[0034] The laser edge removal tool 300 may include a translation mechanism
316 configured to translate the stage 302 and the line of radiation 312
relative to one another. In one embodiment, the translation mechanism 316
is coupled to the stage 302 to move the stage 302 relative to the wave
electromagnetic radiation source 308 and/or the focusing optics 310. In
another embodiment, the translation mechanism 316 is coupled to the wave
electromagnetic radiation source 308 and/or the focusing optics 310 to
move the wave electromagnetic radiation source 308 and/or the focusing
optics 310 relative to the stage 302. In yet another embodiment, the
translation mechanism 316 moves both the wave electromagnetic radiation
source 312 and/or the focusing optics 310, and the stage 302. However,
any suitable translation mechanism may be used, such as a conveyor
system, rack and pinion system, or an x/y actuator, a robot, or other
suitable mechanism.

[0035] The translation mechanism 316 may be coupled to a controller to
control the scan speed at which the stage 302 upon which substrate 202 is
supported and the line of radiation 312 move relative to one another.
Translation of the stage 302 and the line of radiation 312 relative to
one another may be configured to be along the periphery region 210 of the
substrate 202 to focus on removing the films on the substrate edge
without damage to other regions of the substrate 202. In one example, the
translation mechanism 316 moves at a constant speed, of approximately
1000 centimeters per second (cm/s) for a 10 mm to 20 nm wide line, for
example, similar to the width 208 of the periphery region 210 of the
substrate 202. The translation of the stage 302 and the line of radiation
312 relative to one another may be moved with other paths as desired.

[0036] The laser edge removal process, as will be discussed below, removes
unwanted film stack at the periphery region 210 of the substrate 202 and
ensures the cleanliness of the substrate periphery region to facilitate
the subsequent frame bonding process.

Laser Edge Removal Process

[0037] During the laser edge removal process, the line of radiation 312
emitted from the wave electromagnetic radiation source 308 is controlled
to adjust a position of the "spot" of the radiation beam within a
scribing field in the periphery region 210 of the substrate 202.
Depending upon the application, a spot size directed on the substrate 202
may be on the order of microns, such as 1 mm2 within a scanning
field of approximately 60 mm×60 mm, although various other
dimensions are possible depending upon the process regime. "Scanning
filed" is defined as the length of travel of the focused radiation beam
in one dimension parallel to the direction of travel of the sweeping
beam. By controlling a size and position of the scanning field relative
to the substrate 202, the beams (e.g., lasers) are able to effectively
ablate off materials at any location on the substrate 202 while making a
minimal number of passes over the substrate 202. The substrate 202 may be
moved relative to the wave electromagnetic radiation source 308 in either
a transverse (Y) direction or a longitudinal (X) direction using any
conventional approach as discussed above. For example, if the substrate
202 is moved in a longitudinal direction, the wave electromagnetic
radiation module 306 may direct the radiation 312 in the longitudinal,
but opposite direction to the substrate, to form portions or segments of
scribe lines within the scanning field along the periphery region 210 of
the substrate 202. In such a case, the substrate 202 may be supported and
transported by the stage 302 configured to slide within a processing
chamber using, for example, the translation mechanism 316 as discussed
above. The stage 302 may be configured to move the substrate 202 in a
desired direction along with the wave electromagnetic radiation module
306. Alternatively, the stage 302 may be held stationary and the wave
electromagnetic radiation module 306 is controlled to move in a desired
direction or pattern relative to the stage 302. If desired, the stage 302
and the wave electromagnetic radiation module 306 may be both controlled
so as to be movable relative to one another during the laser edge removal
process.

[0038] During the laser edge removal process, a pulse of radiation 312 is
directed through a substantially transparent substrate 202, such as glass
in one exemplary embodiment, to a desired location or depth in one or
more of the films to be ablated. In various embodiments, the radiation
312 may be a pulsed, Q-switched fiber laser operating at a frequency of
about 30 kHz to about 150 kHz and at a wavelength on the order of about
266 nm, 532 nm, or 1064 nm. The layers of material to be ablated, such as
the metal back contact layer 218, a portion of the silicon-containing
film layer 216' sandwiched between the first TCO layer 214 and the back
metal contact layer 218, and the first TCO layer 214 (FIG. 2B), are on
the opposite side of the substrate 202 from the laser, such that the
laser pulses pass through the substrate 202 and ablate the layer(s) on
the opposite side, thus causing the exposed material to ablate up and
away from the substrate 202, which can be extracted by the particle
collector 304 using, for example, vacuum force.

[0039] In order to remove one or more of the films in the periphery region
210 along the side of the substrate 202, a single-pass pulsed laser
thermal ablation process using high pulse energy has been adapted by the
industry to achieve clean material removal in the thermal ablation
regime. With growing number of thin film layers in the tandem junction
design, the film stack thickness is approaching about 3 microns or
greater. As a result, a higher laser pulse energy is typically required
to achieve clean material removal and additional manufacturing
consideration such as throughput. However, it has been reported that the
conventional single-pass laser edge removal on the tandem junction solar
panel often leaves powder residues and visible debris, which may impact
long-term product reliability. In addition, a single-pass laser may limit
the amount of laser power that can be used to ablate off the materials
without damaging the glass, or changing the film properties of the first
TCO (transparent conductive oxide) layer 214 in the neighborhood area,
the use of a stronger laser energy for removing thicker film stacks in
the periphery region 210 of the substrate 202 can result in the back
metal inevitably melting with the silicon-containing film layer 216'
and/or the first TCO layer 214, forming an unwanted alloy at the
periphery region of the substrate which makes the resulting materials
even harder to remove.

[0040] Contrary to the conventional single-pass laser edge removal
process, the present inventors propose a multi-pass laser edge removal
process which has proved effective in removing the film stacks in the
periphery region along the side of the substrate without damaging the
glass or melting the back electrode. In one embodiment, the multi-pass
laser edge removal process includes a two-pass process in which a first
laser pass using a relatively lower power energy is directed to remove
the back metal contact layer 218 and the silicon-containing film layer
216' sandwiched between the first TCO layer 214 and the back metal
contact layer 218, and a second laser pass using a higher power energy is
directed to remove the first TCO layer 214. The energy needed for the
first laser pass should be sufficient to ablate two different types of
layers (i.e., the back metal contact layer 218 and the silicon-containing
film layer 216') sequentially or concurrently while being low enough to
prevent damage or change the properties of the first TCO layer 214
underneath.

[0041] During the first laser pass, the line of radiation 312 emitted from
the wave electromagnetic radiation source 308 penetrates the substrate
202 from the bottom and focuses on the material to be ablated. A laser
pulse of sufficient intensity then heats the target material layers e.g.,
the back metal contact layer 218 and the silicon-containing film layer
216' rapidly and resulting in removal of the materials from the substrate
202. The materials removed from the surface of the substrate 202 are then
extracted by the particle collector 304. Since the back metal and silicon
film stacks inherently offer better heat absorption compared to the first
TCO layer 214, the energy needed for the first laser pass can be lower
and therefore will not melt the back metal contact layer 218 and the
silicon-containing film layer 216' before they can be ablated and will
not change film properties of the first TCO layer 214, nor damage the
glass substrate when passing through the substrate 202.

[0042] Upon removal of the back metal contact layer 218 and the
silicon-containing film layer 216', the first TCO layer 214 is exposed to
air. The second laser pass is then performed to direct the line of
radiation 312 at the exposed first TCO layer 214 and ablate the first TCO
layer 214 from the periphery region 210 of the substrate 202. Since the
first TCO layer 214 has lower heat absorption compared to the back metal
and silicon film stacks and is the only material remaining in the
periphery region of the substrate 202, the energy needed for the second
laser pass can be higher than that of the first laser pass. While being
stronger than the first laser pass, it is contemplated that the highest
pulse energy of the second laser pass is still below the glass damage
threshold.

[0043] The power level for first and second laser passes may be reversed
or adjusted in cases where the wave electromagnetic radiation module 306
is positioned above the substrate 202. In such a case, a relatively lower
pulse energy of the first laser pass may sequentially or concurrently
ablate off the back metal contact layer 218 and the silicon-containing
film layer 216' from the top (i.e., opposite to the bottom of the
substrate), followed by a higher pulse energy of the second laser pass
removing the first TCO layer 214.

[0044] While not shown in the drawing, it is understood that the scribing
line configured to remove unwanted layers during each laser pass is
actually formed of a series of overlapping scribe "dots," each being
formed by a pulse of the radiation 312 directed to a particular position
on the periphery region 210 along the side of the substrate 202. To
ensure acceptable intensity of the ablation process, a spot of sufficient
size, e.g., about 1 mm2 is typically required. In order to form a
continuous line, these dots must sufficiently overlap, such as by about
2% to about 30% by area, for example, between about 5% and about 20%. In
cases where the periphery region 210 of the substrate has a width 208
ranging between about 8 mm and about 15 mm, such as about 12 mm, the wave
electromagnetic radiation module 306 may have to make multiple passes in
order to form multiple overlapping scribing lines where a portion of each
scribing line is overlapped to prevent gaps. In such a case, the scribing
lines may be sufficiently overlapped from each other by about 5% to about
20% by area, such as about 8% to about 15% by area, for example about
10%. The scribing lines may be overlapped in a desired pattern such as a
zig-zag pattern so as to form one or more scribing segments. These
scribing segments may need to be sufficiently overlapped to prevent gaps
while substantially covering the periphery region 210 of the substrate.
In one example where a substrate having a width of about 2.6 m and a
length of about 2.2 m is used, the scribing segments may be sufficiently
overlapped by about 20% to about 40%, such as about 25% to about 35% by
area, for example about 30%, to effectively cover the periphery region of
the substrate while maintaining a high throughput.

[0045] In various embodiments where a rectangular substrate having a width
of about 2.6 m and a length of about 2.2 m is used, several scribing
spots on a spot size of about 1 mm2 may be overlapped at an
exemplary range of overlap percentage as discussed above and moved in a
desired pattern to provide a scribing segment with a length of about 70
mm and a width of about 12 mm. The size of the scribing segment may vary
depending upon the type and field view of the focusing optics 310 used.
For the substrate with a width or length of about 2.6 m or 2.2 m, several
scribing segments may be overlapped at an exemplary range of overlap
percentage as described above to properly cover the entire periphery
region 210 of the substrate 202. During the laser edge removal process, a
line of electromagnetic radiation 312 (FIG. 3) is directed to travel
forward along the 2.6 m or 2.2 m side of the substrate with a relatively
lower laser pulse energy to ablate off the back metal contact layer 218
and the silicon-containing film layer 216', and then the line of
electromagnetic radiation 312 is traveling backward (reverse direction)
along the 2.6 m or 2.2 m side of the substrate with a relatively higher
laser pulse energy to ablate off the TCO layer 214 from the periphery
region 210 of the substrate 202. It is contemplated that the power level
of the electromagnetic radiation and/or the size of the scribing segment
may be adjusted as necessary to repeat the laser edge removal process in
a single traverse over the substrate so as to cover the entire edge of
the substrate.

[0046] In one specific embodiment of a tandem junction silicon solar panel
where the film stack to be removed from the periphery region 210 of the
substrate 202 is composed of a TCO layer of SnO2 at a thickness of
about 1 μm, a silicon-containing film stack at a thickness of about 2
μm, and a metal back contact at a thickness of about 0.2 μm, a
target film stack (i.e., the metal back contact and the
silicon-containing film stack) is removed from a periphery region 210 of
the substrate 202 during a first scan of laser pulses delivered at a
first energy density of about 15 mJ/mm2 to about 75 mJ/mm2, for
example, about 25 mJ/mm2, and a duration in a range of about 1 nsec
to about 3000 nsec, for example, about 35 nsec. Once the target film
stack is removed, the TCO layer is removed from the periphery region 210
of the substrate 202 during a second scan of laser pulses delivered at a
second energy density of about 45 mJ/mm2 to about 120 mJ/mm2,
for example, about 69 mJ/mm2, and a duration in a range of about 1
nsec to about 3000 nsec, for example, about 35 nsec. In one example, the
first scan of laser pulses may be delivered by directing a series of
sequential laser pulses at a spot overlap of about 2% to about 15%, such
as about 5% spot overlap, a line overlap of about 5% to about 15%, such
as about 10% line overlap, and a segment overlap of about 15% to about
45%, such as about 30% segment overlap; and the second scan of laser
pulses may be delivered by directing a series of sequential laser pulses
at a spot overlap of about 10% to about 30%, such as 20% spot overlap, a
line overlap of about 5% to about 20%, such as about 10% line overlap,
and a segment overlap of about 15% to about 45%, such as about 30%
segment overlap.

[0047] While not discussed here, it is contemplated that the wave
electromagnetic radiation module 306, the substrate 202, or a combination
thereof may be moved in a simple rectangular pattern, a zig-zag pattern,
a serpentine pattern, or any desired pattern that would result in the
above-mentioned overlapping spots, lines, or segments of radiation
covering the periphery region 210 of the substrate 202 to be ablated off.
Suitable examples regarding various scribing movement are disclosed in
detail by US Patent Publication No. 2009/0255911, filed Apr. 10, 2009 by
Krishnaswani et al, titled "Laser Scribing Platform and Hybrid Writing
Strategy", which is commonly owned by Applied Materials and which is
incorporated herein by reference.

[0048] One of ordinary skill in the art will appreciate that the power
level of the inventive two-scan process may vary depending upon the film
stack configuration, parameters such as layer thickness, layer doping, or
layer reflectivity, and process regime provided by the laser. While a
two-scan process is discussed, the laser edge removal process may be
repeated as many times as necessary to properly cover the entire edge of
the substrate. Multiple scanning with gradually increased/decreased laser
power is also contemplated depending upon application. Similarly, the
frequency, scan speed, and output current of the laser may be adjusted
during a laser edge removal process in order to control the overlap area.
The amount of the wave electromagnetic radiation module may also affect
the number of scanning passes required to process the periphery region.
For example, if there are multiple laser scanning devices available in
the system to ablate off one of these segments, the substrate 202 can
make fewer passes through the module 306.

[0049]FIG. 4 depicts a cross sectional view of the substrate 202 after
the laser edge removal process is performed. After the laser edge removal
process is complete, the films previously located at the periphery region
210 of the substrate 202 are removed. Optionally, a portion of the
substrate 202 may also be removed, for example, a depth 402 between about
20 μm and about 40 μm of the substrate surface can be removed.
Thus, improved methods for removing one or more film stacks disposed at a
substrate edge are provided. The multi-pass laser edge removal processes
according to the present invention advantageously increase clearness of
the solar cell devices and accuracy of removing a film stack at a
periphery region along the side of a substrate without having issues with
residue powders or formation of unwanted alloys before they can be
removed, thereby providing a good seal surface for the substrate to
facilitate subsequent bonding and packaging processes.

[0050] While the foregoing is directed to embodiments of the present
invention, other and further embodiments of the invention may be devised
without departing from the basic scope thereof, and the scope thereof is
determined by the claims that follow.